Previous coded modulation in digital video broadcasting transmission mainly comprised a combination of binary channel coding with non-binary modulation. Concatenated encoders employing an inner coding block residing near the transmission channel and an outer coding block residing near the data input are known in the art. Some concatenated encoders have also used interleaving devices between the inner and outer coding blocks to mitigate the effects of burst errors in the transmission channel. A block code such as a Reed-Solomon code is generally used as an outer code while codes such as Trellis Coded Modulation are usually used as an inner code. Binary Low Density Parity Check codes have also been used as inner codes with the outer code being either Reed-Solomon or BCH coding.
In the European DVB-T standard for terrestrial broadcast, a concatenated channel coding scheme is applied. It uses Reed-Solomon (RS) code (204,188) as an outer code, and a convolutional code as an inner code. A convolutional interleaver positioned between the outer and inner code elements is used to reduce the effects of long sequences of channel errors. In the United States the terrestrial broadcast standard employs an eight level vestigial side band (8-VSB) modulation technique. This technique also uses a concatenated scheme that features RS (207,187) as an outer code with Trellis Coded 8-Level Vestigial Side-Band (8-VSB) modulation. Again, a convolutional interleaver is positioned between the outer and inner code elements. The ISDB Japanese broadcast standard has a channel coding scheme that is similar to that of DVB-T.
Binary coding is well suited for binary modulation. Because of increased channel capacity requirements, non binary quadrature amplitude modulations, such as 16QAM, 64QAM, and 256QAM, have been used in digital video broadcasting transmission systems. When binary channel coding is used with non-binary memoryless modulation, bitstreams must be mapped into symbols before modulation at the transmitter side. Similarly, on the receiver side, conversion of modulation symbol likelihoods into code bit probabilities is needed before decoding decisions can be made.
Standard theoretical analysis shows that a significant loss in performance may be caused by bit-to-symbol and symbol-to-bit conversions. To get better performance, binary extrinsic probabilities generated by the channel decoder are fed back to the de-mapping section in order to close the iterative detection/decoding loop on the receiver side. On the other hand, when the channel code alphabet is matched to the modulation alphabet, the channel likelihoods are directly processed by the decoder without any information loss, and there is no need to iterate between demodulation and decoding.
Binary Low Density Parity Check (LDPC) codes are recognized as capacity approaching codes for various types of channels when the size of the codeword tends to infinity. However, there are circumstances under which the limitations of binary LDPC codes have a negative effect on performance. Using small or moderate block lengths or combining binary LDPC coding with non binary modulation highlight the limitations of binary LDPC coding.
One of the goals of the invention is to overcome the problems encountered when small or medium block lengths are used. Another goal is to reduce the problems encountered while using binary LDPC coding as described.
Trellis coded modulation (TCM) is known to increase the Euclidean distance between symbols in a constellation. In prior art TCM systems the constellation that is generated is directly applied to an in (I) phase and a quadrature (Q) phase IQ modulator for application to a transmission channel.